Hydroperoxidase activity

Nunez-Delicado E, Sojo MM, Sanchez-Ferrer A and Garcia-Carmona F. 1999. Hydroperoxidase activity of lipoxygenase in the presence of cyclodextrins. Arch Biochem Biophys 367(2) 274-280. 

Figure 5 Conversion of acetaminophen to its reactive free radical by co-oxidation mediated by the hydroperoxidase activity of prostaglandin H synthase-catalyzed reduction of prostaglandin Ga (PGGa) to prostaglandin Ha (PGHa).

Radical production from peroxidase-like systems, for example from the peroxide-supported oxidation of amino compounds catalyzed by protohemin and metmyo-globin, has received attention [174]. Both hydrogen peroxide and hydroperoxides (e.g., r-butyl hydroperoxide and cumene hydroperoxide) can be effective sources of oxidizing equivalents for these reactions. Since the enzyme prostaglandin synthase contains both cyclooxygenase and hydroperoxidase activities, either a substrate for the cyclooxygenase or an added hydroperoxide will support the catalyzed oxidation of substrates [175]. 

Bioactivation to a free radical intermediate has been implicated in the teratological mechanism for a number of xenobiotics, including phenytoin and structurally-related AEDs, benzo[a]pyrene, thalidomide, methamphetamine, valproic acid, and cyclophosphamide (Fantel 1996 Wells et al. 2009 Wells and Winn 1996). Unlike in the case of most CYPs, the embryo-fetus has relatively high activities of PHSs and lipoxygenases (LPOs), which via intrinsic or associated hydroperoxidase activity can oxidize xenobiotics to free radical intermediates (Fig. 10) (Wells et al. 2009). These xenobiotic free radical intermediates can in some cases react with double bonds in cellular macromolecules to form covalent adducts, or more often react directly or indirectly with molecular oxygen to initiate the formation of potentially teratogenic reactive oxygen species (ROS). 

Another effect of the heme cofactor is in the inactivation of the enzyme. When the bovine enzyme is mixed with hematin or hemoglobin, enzyme activity is lost rather rapidly. The rate of enzyme inactivation depends on the heme/enzyme ratio. As the amount of heme is raised beyond the equimolar quantity, the inactivation occurs faster. The half life of the enzyme is approximately 30 min when incubated with 5-fold excess of hematin or of hemoglobin at 24°C, pH 7.4. All the active heme compounds including the mangano analogue inactivate the enzyme. Both the cyclooxygenase and hydroperoxidase activities are lost in parallel [27]. A similar effect is observed with the ovine enzyme, and peroxide is presumed to participate in the heme-induced inactivation [39]. Such an enzyme-destroying action of heme hampers the purification and kinetic studies of the enzyme. Fortunately, however, the enzyme is protected from the heme-induced inactivation by the simultaneous presence of tryptophan, epinephrine, hydroquinone [27], phenol [27,39] and other compounds, all of which are cofactors for the hydroperoxidase reaction (see below). 

The purified enzyme from bovine vesicular gland catalyzes the conversion of PGG to PGH in the presence of tryptophan. If tryptophan is replaced by glutathione, there is essentially no conversion of PGG to PGH. The purified enzyme is free of glutathione peroxidase activity as tested with glutathione and cumene hydroperoxide [38]. Thus, glutathione peroxidase is not involved in the conversion of PGG to PGH so far as it is catalyzed by the PG hydroperoxidase activity of PG endoper-oxide synthetase. 

Prostaglandins (PCs) are bioactive compounds involved in various symptoms associated with inflammation. The first step in prostaglandin biosynthesis is catalysed by an enzyme called cyclooxygenase or prostaglandin endoperoxide synthase (COX EC 1.14.99.1). COX is a bifunciional enzyme with both cyclooxygenase activity and PG hydroperoxidase activity [1,2]. The enzyme catalyses the conversion of arachidonic acid into PGGj by the former activity. Subsequently,... 

The oxidation of catecholamines by the hydroperoxidase activity of lipoxygenase (EC 1.13.11.12) is documented by Rosei et al. (1994) and NOnez-Delicado et al. (1996). o-Diphenols are easily studies spectrophotometrically since, when oxidised, they render coloured compounds, quinones, or their corresponding aminochromes. In the case of isoprenaline the maximum of the oxidation product was developed at 490 nm (NOnez-Delicado et al. 1999), which corresponds to that of the aminochrome product (Jim nez et al. 1985, NOnez-Delicado et al. 1996). 

One single type of assay cannot fill the needs of all hydroperoxidase activity measurements. In some cases simplicity of method and apparatus is the foremost requirement. This applies to most research in what we may call applied enzjnnology, where experiments are usually carried out with whole tissues or homogenates. The activity of hydroperoxidases has been measured in relation to cancer, vegetable storage, detection of bacteria, radiation research, and many other fields including the study of convenient enzyme models. In these cases a simple and reliable test is required. 

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